Flat Panel radiation Detectors (FPDs) are the current standard in static and dynamic radiography clinical applications. Static applications include cranial, skeletal, podiatric, thoracic, and lung exposures. Dynamic (Fluoroscopy) clinical applications include gastrointestinal tract, urogenital tract, lymphography, endoscopy, myelography, venography, digital angiography, and digital subtraction angiography.
Further, FPDs quickly penetrate new clinical fields such as dental imaging, minimal invasive surgery, and mammography. Further, they are widely used in non-medical application such as non-distractive testing's (NDT) and security screening.
All radiological systems, both medical and industrial, consist of a radiation source (High Voltage (HV) generator and X-ray tube), and of a radiation detector, such as FPD. The imaged object is positioned between the radiation source and the detector. The object absorbs part of the X-ray radiation. The detector's signal is responsive to the unabsorbed radiation, and is sent to a viewing station for further processing and display.
In an X-ray system, control of the scan operation is a combined task of the generator and the FPD, and it consists of mutual sending of pulses between them, pulses that are designated “prep request” and “X-ray enable”. The operation is initiated by the human operator who presses a button (or a pedal), which triggers the X-ray generator to start a preparation phase. The generator then sends a “prep-request” pulse to the FPD and waits for an “X-ray enable” response of the FPD before starting the actual radiation. Upon sending the “X-ray enable” pulse, the FPD starts acquiring the signal.
The FPD acquisition time is set to be always larger than the expected X-ray pulse: In a case of dynamic X-ray system, where the scan consists of a long sequence of low-dose X-ray pulses, the FPD acquisition timing consists of train of acquisition pulses, each such acquisition pulse is synchronized with a corresponding X-ray pulse, and each is longer than the corresponding X-ray pulse.
In static X-ray systems, the acquisition period is controlled by Automatic Exposure Control (AEC) sub-system. The AEC sub-system used in the art typically uses of up to five small radiation-detection panels mounted between the patient and front of the FPD. The AEC outputs signals, each proportional to the radiation impinging on one of said small radiation-detection panels, are used for controlling the High voltage generator powering the X-Ray tube. The generator is instructed by the user to select one or more of said AEC outputs signals and to apply predefined function on them for obtaining a single value.
For example, in chest radiography, the generator uses signal of the two upper AEC small radiation-detection panels, averages them, and uses the average as the single value. Said single value is integrated, and when the integral crosses a predefined threshold value, the HV generator stops, and the X-ray tube stops producing radiation. Therefore, the AEC sub-system is being operated in “slave” mode only, where the generator collects its signal and performs the required processing for obtaining a single time-increasing value, for comparing it to a preset threshold and stopping the radiation once the threshold value is reached. One reason for this setup is that the AEC sub-system is built as separate unit, mounted in front of the FPD. Thus, any further electronics or electronic board will block the beam and interfere with the proper FPD operation.
Focusing again on the FPD, it consists of as many as millions pixels, the resulting signal of each of the pixels is a function of the radiation flux on that pixel. Said function depends on the physical characteristics of the individual pixel, which are: the sensitivity, defined as the radiation-to-electrical energies conversion factor, the dark signal, and inherent non-linearity. Consequently, for obtaining proper image of the scanned object, the image has to be compensated for the variation in physical characteristics of each pixel.
Typically, a set of three corrections is utilized for each acquired image, either static or dynamic: Offset (dark signal) removal, Gain correction and Defective-pixels replacement. Dark signal is taken close to the clinical scan: either closely before, or right after, and the resulting matrix is kept on the FPD memory or in the host system. Gain correction factors are evaluated by taking a uniform (no object) image and assigning correction factor for each of the pixel which inversely proportional to its measured value.
For the defective pixels, FPD systems typically use a defective-pixel correction mean, consisting of an algorithm for detecting these defective pixels. Said algorithm consists of testing the above described physical characteristics and listing the pixels with exceptional characteristics, exceeding pre-set thresholds. Further, said defective-pixel correction mean consists of an algorithm for actually correcting the image values associated with the defective pixels. Typically, said algorithm consists of replacing the data of the defective pixels by some average of neighboring-pixels data. These three corrections consist of storing large correction matrices and operating corrections in real-time. Therefore, the correction operations are done in the host systems, not in the FPD.
FPDs are built to fit into slim geometries. For example, FPDs are being inserted in film-cassette and Compute Radiography (CR) Buckys, for upgrading analog or CR rooms into digital ones.
Many prior inventions describe modifications of the above-described calibration scheme:
JP2000126162 discloses a radiographic image processing system that outputs two images: One with corrected pixel defects and one with uncorrected pixel defects.
JP2000132662 discloses a radiographic image processing system that gives a warning when the number of defective pixels exceeds a given value, and provides information related to the defect.
JP2008229102 discloses system which enables radiographic imaging to be continued for a while after occurrence of pixel defects that may lower image quality and minimizes adverse effects of the pixel defects.
U.S. Pat. No. 7,203,279 discloses radiographic mode designator which designates a non-standard radiographic mode and a signal corrector which uses defect information stored in one of non-standard image defect information memories for correcting X-ray detection signals outputted from the FPD, thus, making it unnecessary to collect output signals for pixel defect information acquisition from the FPD all over again.
JP2009201586 discloses radiographic apparatus capable of executing appropriate afterimage correction of a radiation image according to the state of an afterimage based on the charge accumulation time in a FPD at each pixel position.
JP2009219691 discloses radiographic apparatus which is correctly discriminating whether an image defect on a detected radiation image is an image defect due to dust or an image defect due to the defective pixel of an FPD, provides a defect discriminating method according to whether the pixel is defective both in dark image and flat (no object) image or only in the flat image.
However, all the above referenced inventions assume that at least part of the correction scheme is made in the host system. This implies installing specialized boards and/or software in the host system, thus making installation of the system host-dependent, and obstructs the upgrading of existing X-ray rooms.
With respect to the AEC sub-system:
U.S. Pat. No. 5,617,462 discloses AEC mechanism, e.g., CCD camera, which provides outputs for a microprocessor for analyzing them and for adjusting the X-ray technique rapidly, thus reducing exposure time of X-rays.
U.S. Pat. No. 6,233,310 discloses AEC system which combines a patient model with a closed loop brightness control, using a parameter that does not affect image quality.
U.S. Pat. No. 7,194,065 discloses AEC which compensates for the varying of distances: The distance from the X-ray source and the patient and the distance from the X-ray source and the AEC.
Clearly, all the above inventions assume that the AEC is mounted in front of the FPD or adjacent to it. Therefore, the AECs consist of panels for dose measurements that are virtually almost transparent to the radiation. The rest of the AEC circuitry, which includes preamplifiers, processing electronic and memory boards, is located elsewhere. This fact has no drawback when referring to a stand-alone system where the FPD, AEC and the accompanying preamplifiers, electronic boards and cablings are all enclosed in Bucky mean, which is also provided by the same manufacturer. However, if installed in existing system, the described AEC systems require additional space, which might be scarce in various systems, for example, in portable FPD, in mobile systems and in upgrading of existing film- or CR-systems.